Measurement of radioactive emissions downhole is a well established, widely used technique for monitoring the position and movement of fluids in the well or various aspects of the formation. For example, natural gamma ray logs have provided an important contribution to the evaluation of downhole reservoirs. The naturally occurring radioactive elements potassium (K), uranium (U), and thorium (Th) are the sources of the gamma rays typically counted in such gamma ray logging. Potassium, thorium and uranium have different depositional properties, exhibit different solubility characteristics, and respond differently to diagenetic processes. Therefore, valuable geological and physical parameters, and important information for assisting in well log analysis and reservoir production can be obtained from both absolute and relative K, U, and Th concentrations.
In addition to measurement of gamma rays from naturally occurring sources, radioactive tracers are widely used in subsurface applications for identifying the presence of particular fluids or other additives. Such fluid tracers are for the most part used in marker applications such as identifying communicating zones, cement tops, and hydraulic fractures.
Gamma ray spectroscopy logging tools measure the energy as well as the intensity of the gamma rays, thereby allowing extraction of significantly more information from the log. Measurements made with spectral gamma ray tools have gained wide acceptance because the contributions of two or more isotopes with distinct gamma ray signatures can be determined simultaneously. Hence, spectral gamma ray logging tools are used to distinguish the gamma rays emitted by naturally occuring radioactive elements in the formation from those emitted by individual radioactive tracer(s) introduced in the course of various downhole operations to assist in monitoring the effectiveness of such operations.
Various analysis techniques have been developed in the quest to extract maximum information from gamma ray spectroscopy data. These analysis techniques include the spectrum stripping method originally used and, lately, calibration of the spectroscopy tools to gamma ray response "fingerprints" that enable the use of a weighted-least-squares algorithm. The latter technique allows the use of Compton down-scattering information to be used more effectively to extract additional information, such as estimation of the diameter of a column of radioactive material.
A Compton ratio is defined as a count rate in the high energy part of the gamma ray spectrum (where Compton downscattering contributions are small) divided by a count rate in the low energy portion of the spectrum (where Compton downscattering contributions are greatest). The Compton ratio can be calibrated to give the average diameter of a column of material that contains any radioactive material that emits a distinct gamma ray signature. Furthermore, this concept can be extended to the simultaneous use of more than one gamma-ray-emitting radionuclide. In the case of multiple radioactive isotopes, the weighted-least-squares algorithm is used to generate a borehole and formation component for each isotope, which in turn are summed to yield a composite spectrum for each isotope. The Compton ratio for each isotope thus can be generated on the basis of this composite spectrum.
When a radioactive tracer is to be used to quantitatively measure the presence and thickness of a cement column around a casing string, it is desirable to mix a sufficient amount of tracer uniformly into the cement such that the signal emitted by that tracer is readable through the casing once the cement has been emplaced. The well is logged immediately after cementing with a spectral gamma ray tool. Because the tool can discriminate between gamma rays of different energies, the spectral log can distinguish the tracer from all other gamma rays present. This selectivity means that the activity of the radioisotopes need not overwhelm the formation signal, but rather can be logged at activities only somewhat greater than the natural background radiation levels.
In conventional operations in which a radioactive tracer is used, it is preferable to clean out the hole before logging in order to minimize the presence in the borehole of extraneous radioactive material that would otherwise confound measurement of the tracer signal. Likewise, it is desirable to utilize radioactive tracers giving gamma ray signals that are easily separated from those of naturally occurring radioactive isotopes. Hence, it is typical that the radioactive tracer that is added to the cement be one that is not naturally occurring when it is desired to use gamma ray spectroscopy to determine the presence or thickness of cement.
Artificially generated radioisotopes are disadvantageous, however, in that they may pose significant environmental, health and safety risks. Each such potential tracer emits radiation, and Federal regulations govern handling and deployment of virtually all radioactive materials. Hence, these disadvantages would be avoided if the analysis could be performed without the use of non-naturally occurring radioactive tracers. Thus, it would be preferable to use elements or isotopes that are already present in either the cement or formation as a basis for indicating the presence and thickness of the cement.